Protein kinase C activation down-regulates natriuretic peptide receptor C expression via transcriptional and post-translational pathways

C Type Natriuretic Peptide Receptor Activation Inhibits Sodium Channel Activity in Human Aortic Endothelial Cells by Activating the Diacylglycerol-Protein Kinase C Pathway

The C-type natriuretic peptide receptor (NPRC) is expressed in many cell types and binds all natriuretic peptides with high affinity. Ligand binding results in the activation or inhibition of various intracellular signaling pathways. Although NPRC ligand binding has been shown to regulate various ion channels, the regulation of endothelial sodium channel (EnNaC) activity by NPRC activation has not been studied. The objective of this study was to investigate mechanisms of EnNaC regulation associated with NPRC activation in human aortic endothelial cells (hAoEC). EnNaC protein expression and activity was attenuated after treating hAoEC with the NPRC agonist cANF compared to vehicle, as demonstrated by Western blotting and patch clamping studies, respectively. NPRC knockdown studies using siRNA's corroborated the specificity of EnNaC regulation by NPRC activation mediated by ligand binding. The concentration of multiple diacylglycerols (DAG) and the activity of protein kinase C (PKC) was augmented after treating hAoEC with cANF compared to vehicle, suggesting EnNaC activity is down-regulated upon NPRC ligand binding in a DAG-PKC dependent manner. The reciprocal cross-talk between NPRC activation and EnNaC inhibition represents a feedback mechanism that presumably is involved in the regulation of endothelial function and aortic stiffness.

Divergent effects of a designer natriuretic peptide CD-NP in the regulation of adipose tissue and metabolism

Objective

Obesity is defined as an abnormal increase in white adipose tissue (WAT) and is a major risk factor for type 2 diabetes and cardiovascular disease. Brown adipose tissue (BAT) dissipates energy and correlates with leanness. Natriuretic peptides have been shown to be beneficial for brown adipocyte differentiation and browning of WAT.

Methods

Here, we investigated the effects of an optimized designer natriuretic peptide (CD-NP) on murine adipose tissues in vitro and in vivo.

Results

In murine brown and white adipocytes, CD-NP activated cGMP production, promoted adipogenesis, and increased thermogenic markers. Consequently, mice treated for 10 days with CD-NP exhibited increased “browning” of WAT. To study CD-NP effects on diet-induced obesity (DIO), we delivered CD-NP for 12 weeks. Although CD-NP reduced inflammation in WAT, CD-NP treated DIO mice exhibited a significant increase in body mass, worsened glucose tolerance, and hepatic steatosis. Long-term CD-NP treatment resulted in an increased expression of the NP scavenging receptor (NPR-C) and decreased lipolytic activity.

Conclusions

NP effects differed depending on the duration of treatment raising questions about the rational of natriuretic peptide treatment in obese patients.

•

The optimized designer natriuretic peptide CD-NP promotes adipogenesis.

•

Duration of treatment is decisive: short-term promotes browning whereas long-term treatment exacerbates obesity and diabetes.

•

Long-term CD-NP treatment reduces WAT inflammation and increases adiponectin expression.

Identification, regulation and anti-proliferative role of the NPR-C receptor in gastric epithelial cells

Evidence suggests that functional atrial natriuretic peptide (ANP) receptors occur in surface gastric mucosal epithelial cells. To evaluate functional aspects of ANP in a model of these cells we examined the expression of natriuretic peptide receptors (NPR) subtypes A and C in the non-transformed rat gastric mucosal epithelial cell line RGM1. Transcripts for NPR-A and NPR-C were detected in RGM1 cells by RT-PCR. However, only NPR-C protein was detected by Western blot and immunohistochemical analyses. Specific saturable binding of (125)I-ANP to RGM1 cells revealed a single class of high affinity binding sites (K (d) = 208 +/- 71pM, B (max) = 110,000 +/- 14,000 sites/cell, Hill coefficient = 0.97 +/- 0.05). ANP (IC(50) 130 +/- 47pM), BNP (IC(50) 716 +/- 26 pM), CNP (IC(50) 356 +/- 85pM) and C-ANP (IC(50) 134 +/- 13pM), a specific ligand for NPR-C, effectively displaced (125)I-ANP binding. Cross-linking of (125)I-ANP to cells labeled predominantly a protein of 66,000 Da. These data suggest that (125)I-ANP binding was primarily to NPR-C. ANP and C-ANP inhibited forskolin- and prostaglandin E(2) (PGE(2))-stimulated cAMP in a PTx-sensitive fashion. PGE(2), transforming growth factor-+/-1 (TGF-+/-1), forskolin, 8-bromo-cyclic AMP, and phorbol-12-myristate-13-acetate (PMA) caused a dose-dependent decrease in specific (125)I-ANP binding, whereas epidermal growth factor (EGF), 8-bromo-cyclic GMP and 4+/--phorbol didecanoate had no effect. PGE(2), forskolin, TGF-+/-1 and PMA significantly decreased (125)I-ANP B (max) values, NPR-C protein and steady-state NPR-C transcript levels. H89, a protein kinase A inhibitor, blocked the reduction of NPR-C mRNA produced by both forskolin and PGE(2.) GF109203X, a protein kinase C inhibitor, abolished the PMA-induced decrease in NPR-C transcripts but only partially blocked that produced by TGF-+/-1. RGM1 cells exhibited a dose-dependent decrease in both DNA synthesis and cell proliferation when cultured in the presence of ANP or C-ANP. These findings indicate that RGM1 cells express functional NPR-C receptors that can influence RGM1 cell proliferation and are down-regulated by PGE(2) and TGF-+/-1.

Modulation of ANP-C receptor signaling by arginine-vasopressin in A-10 vascular smooth muscle cells: role of protein kinase C

We have previously shown that pretreatment of A-10 vascular smooth muscle cells (VSMC) with angiotensin II (Ang II) attenuated atrial natriuretic peptide receptor-C (ANP-C)-mediated inhibition of adenylyl cyclase without altering [125I]ANP binding. In the present studies, we have investigated the modulation of ANP-C receptor signaling by arginine-vasopressin (AVP). Pretreatment of A-10 VSMC with AVP for 24h resulted in a reduction in ANP receptor binding activity by about 50% (B(max); control cells, 22.9+/-2.5 fmol/mg protein, AVP-treated cells, 11.4+/-1.2 fmol/mg protein). In addition, the expression of ANP-C receptor as determined by immunoblotting was also decreased by about 50% by AVP treatment, which was prevented by GF109203X, an inhibitor of protein kinase C (PKC). The decreased expression of ANP-C receptor was reflected in an attenuation of ANP-C receptor-mediated inhibition of adenylyl cyclase. C-ANP(4-23) [des(Gln(18),Ser(19),Gln(20),Leu(21),Gly(22))ANP(4-23)-NH(2)], a ring deleted peptide of ANP that interacts specifically with ANP-C receptor, inhibited adenylyl cyclase activity by about 30% in control cells, which was completely attenuated in AVP-treated cells. This attenuated inhibition was significantly restored by GF 109203X. In addition, AVP treatment augmented the levels of Gialpha-2 and Gialpha-3 proteins; however, the Gi functions were completely attenuated. The increased expression of Gialpha proteins induced by AVP was inhibited by GF109203X as well as by actinomycin D treatments. In addition, AVP treatment also enhanced the expression of Gsalpha protein and Gsalpha-mediated stimulation of adenylyl cyclase by GTPgammaS, N-ethylcarboxamide adenosine (NECA), and forskolin (FSK), whereas the levels of Gbeta were not altered by AVP treatment. These results indicate that AVP-induced PKC signaling may be responsible for the down-regulation of ANP-C receptor that results in the attenuation of C-ANP(4-23)-mediated inhibition of adenylyl cyclase activity, and suggest a cross-talk between vasopressin V(1) and ANP-C receptor-mediated signaling pathways.

cGMP-phosphodiesterase activity is up-regulated in response to pressure overload of rat ventricles

Although expression of natriuretic peptides in cardiac tissues is up-regulated in response to pressure overload, no significant change in cGMP level in hypertrophied ventricles was observed. Activities of two cyclic nucleotide phosphodiesterase (PDE) isoforms, Ca2+/calmodulin-stimulated PDE (PDE1) and cGMP-stimulated PDE (PDE2), were significantly higher in rat left ventricles 14 days after aortic banding. The absence of significant changes in PDE1A and PDE2A mRNA levels indicated that the two PDE activities were post-transcriptionally up-regulated. These results suggested that the increased cGMP-PDE activity in response to pressure overload plays an important role in neutralizing cGMP action in cardiac tissue.

Modulation of ANP-C receptor signaling by endothelin-1 in A-10 smooth muscle cells

We have previously shown that pretreatment of A-10 smooth muscle cells (SMC) with angiotensin II (Ang II) attenuated atrial natriuretic peptide (ANP) receptor-C (ANP-C)-mediated inhibition of adenylyl cyclase without altering (125)I-ANP binding. In the present studies, we have investigated the modulation of ANP-C receptor signaling by endothelin-1 (ET-1). Pretreatment of A-10 SMC with ET-1 for 24 h attenuated the expression of ANP-C receptor by about 60% as determined by immunoblotting which was reflected in attenuation of ANP-C-receptor-mediated inhibition of adenylyl cyclase. C-ANP(4-23) [des(Gln(18),Ser(19),Gln(20),Leu(21),Gly(22))ANP(4-23)-NH(2)], a ring-deleted peptide of ANP that interacts specifically with ANP-C receptor, inhibited adenylyl cyclase activity in a concentration-dependent manner with an apparent K(i) of about 1 nM in control cells. The maximal inhibition observed was about 30% which was almost completely attenuated in ET-1-treated cells. In addition, Ang II- and oxotremorine-mediated inhibitions of adenylyl cyclase were also attenuated by ET-1 treatment; however, the expression of Gialpha-2 and Gialpha-3 proteins and not of Gsalpha and Gbeta proteins was augmented by such treatment. The increased expression of Gialpha-2 and Gialpha-3 proteins by ET-1 treatment was inhibited by actinomycin D treatment (RNA synthesis inhibitor). On the other hand, the Gsalpha-mediated effects of some agonists on adenylyl cyclase activity were significantly decreased by ET-1 treatment. These results suggest that ET-1-induced downregulation of ANP-C receptor and not the overexpression of Gi proteins may be responsible for the attenuation of C-ANP(4-23)-mediated inhibition of adenylyl cyclase activity. From these studies it may be suggested that the downregulation of ANP-C receptors by increased levels of endothelin in vivo may be one of the possible mechanisms for the pathophysiology of hypertension.

Tyrosine kinase receptor activation inhibits NPR-C in lung arterial smooth muscle cells

We have previously demonstrated that expression of the atrial natriuretic peptide (ANP) clearance receptor (NPR-C) is reduced selectively in the lung of rats and mice exposed to hypoxia but not in pulmonary arterial smooth muscle cells (PASMCs) cultured under hypoxic conditions. The current study tested the hypothesis that hypoxia-responsive growth factors, fibroblast growth factors (FGF-1 and FGF-2) and platelet-derived growth factor-BB (PDGF-BB), that activate tyrosine kinase receptors can reduce expression of NPR-C in PASMCs independent of environmental oxygen tension. Growth-arrested rat PASMCs were incubated under hypoxic conditions (1% O2) for 24 h; with FGF-1, FGF-2, or PDGF-BB (0.1-20 ng/ml for 1-24 h); or with ANG II (1-100 nM), endothelin-1 (ET-1, 0.1 microM), ANP (0.1 microM), sodium nitroprusside (SNP, 0.1 microM), or 8-bromo-cGMP (0.1 mM) for 24 h under normoxic conditions. Steady-state NPR-C mRNA levels were assessed by Northern blot analysis. FGF-1, FGF-2, and PDGF-BB induced dose- and time-dependent reduction of NPR-C mRNA expression within 1 h at a threshold concentration of 1 ng/ml; hypoxia, ANG II, ET-1, ANP, SNP, or cGMP did not decrease NPR-C mRNA levels in PASMCs under the above conditions. Downregulation of NPR-C expression by FGF-1, FGF-2, and PDGF-BB was inhibited by the selective FGF-1 receptor tyrosine kinase inhibitor PD-166866 and mitogen-activated protein/extracellular signal-regulated kinase inhibitors U-0126 and PD-98059. These results indicate that activation of tyrosine kinase receptors by hypoxia-responsive growth factors, but neither hypoxia per se nor activation of G protein-coupled receptors, inhibits NPR-C gene expression in PASMCs. These results suggest that FGF-1, FGF-2, and PDGF-BB play a role in the signal transduction pathway linking hypoxia to altered NPR-C expression in lung.

Protein kinase C regulates transcription of the human guanylate cyclase C gene

Guanylate cyclase C is the receptor for the bacterial heat-stable enterotoxins and guanylin family of peptides, and mediates its action by elevating intracellular cGMP levels. Potentiation of ligand-stimulated activity of guanylate cyclase C in human colonic T84 cells is observed following activation of protein kinase C as a result of direct phosphorylation of guanylate cyclase C. Here, we show that prolonged exposure of cells to phorbol esters results in a decrease in guanylate cyclase C content in 4beta-phorbol 12-myristate 13-acetate-treated cells, as a consequence of a decrease in guanylate cyclase C mRNA levels. The reduction in guanylate cyclase C mRNA was inhibited when cells were treated with 4beta-phorbol 12-myristate 13-acetate (PMA) in the presence of staurosporine, indicating that a primary phosphorylation event by protein kinase C triggered the reduction in RNA levels. The reduction in guanylate cyclase C mRNA levels was not due to alterations in the half-life of guanylate cyclase C mRNA, but regulation occurred at the level of transcription of guanylate cyclase C mRNA. Expression in T84 cells of a guanylate cyclase C promoter-luciferase reporter plasmid, containing 1973 bp of promoter sequence of the guanylate cyclase C gene, indicated that luciferase activity was reduced markedly on PMA treatment of cells, and the protein kinase C-responsive element was present in a 129-bp region of the promoter, containing a HNF4 binding element. Electrophoretic mobility shift assays using an oligonucleotide corresponding to the HNF4 binding site, indicated a decrease in binding of the factor to its cognate sequence in nuclear extracts prepared from PMA-treated cells. We therefore show for the first time that regulation of guanylate cyclase C activity can be controlled at the transcriptional level by cross-talk with signaling pathways that modulate protein kinase C activity. We also suggest a novel regulation of the HNF4 transcription factor by protein kinase C.